With recent development of mobile communication and the Information-Electronic Industry, higher capacity, smaller and lighter lithium secondary batteries are increasingly in demand. However, with diversification of functions of the portable or mobile electronic equipment, which is thereby concomitantly accompanied by increased energy consumption of the equipment, there is also a strong need for realization of higher power and capacity of the batteries. Therefore, a great deal of research and study has been widely conducted to increase C-rate characteristics and capacity of the battery cells.
However, there is the presence of reciprocal relationship between C-rate characteristics and capacity of the battery cell. That is, when a loading amount or electrode density of the cell is increased in order to improve cell capacity, this attempt usually results in deterioration of C-rate characteristics of the battery cell.
Upon taking into consideration ionic conductivity of active materials, lithium secondary batteries, as shown in FIG. 1, are needed to maintain the electrode porosity over a predetermined level. Whereas, if the electrode is rolled at a high-rolling reduction rate in order to achieve increased loading amount or electrode density, the electrode porosity is excessively decreased, as shown in FIG. 2, which in turn leads to a rapid decrease in the C-rate. Further, when the same active materials having different particle diameters are used as an electrode active material, it is possible to accomplish a high electrode density by moderate rolling, but the electrode porosity is strikingly decreased as shown in FIG. 3, thereby leading to significant decreases in the C rate.
Therefore, although it is important to maintain appropriate porosity in order to meet a proper level of C rate characteristics, the thus-maintained void remains as a dead volume where the electrode is free of the active materials.
Secondary batteries must maintain a given level of C-rate suited for the corresponding uses thereof. In particular, secondary batteries for use in electrically-driven tools that require elevated power or secondary batteries for use in electric vehicles (EVs) and hybrid electric vehicles (HVs) require significantly higher C-rate. Consequently, in order to increase the battery power, there is a strong need for the development of a lithium secondary battery having improved C-rate characteristics in conjunction with maximized cell capacity.